The invention relates to a holder for accurate positioning of a workpiece in the working area of a machine tool, especially an erosion machine (e.g., electrical discharge machine or xe2x80x9cEDMxe2x80x9d). The holder includes attachment or fastening elements for attaching or fastening the holder for holding the workpiece to a fixed carrier structure and holding parts for holding the workpiece.
In the electromachining process of a workpiece it is necessary to remove from the immediate working area all of the ultrafine particles removed from the workpiece in order not to degrade the desired accuracy of the machining. To remove particles there is usually a water-based rinse fluid, aimed at the workpiece in the working arm. As the cutting speed increases, larger quantities of particulate powder have to be removed. Its removal requires that the rinse fluid pressure be increased.
The erosion process itself does not apply any force to the workpiece, because the erosion electrode, for example in the form of a cutting wire, does not make contact with the workpiece so that the workpiece position is not affected. In contrast, the increased, normally not exactly constant rinse fluid pressure acts directly on the workpiece and sets the workpiece oscillating in its holder, a state that degrades the accuracy of the workpiece position and thus the results of the workpiece machining.
To overcome this problem it is quite conceivable that the mass of the holder be enlarged beyond the conventional strength requirements. Then, however, the holder is unwieldy and reduces the working area.
The German patent document DE-A-41 39 272 discloses a sliding block with a supporting element, which is surrounded bypacking material made of non-metallic material, which is supposed to dampen sound and vibrations.
Various embodiments of the present invention are based on the problem of developing a holder which is insensitive to the rinse fluid pressure and does not restrict the working area. However, such pressure fluctuations, which act on the workpiece, can also occur during other machining processes of the workpiece, such as for example during the cutting or lapping operation of the workpiece. Such pressure fluctuations can be the reason for the workpiece to vibrate; and said vibration can propagate into said workpiece""s holder and into the support structure. Therefore, the holder may be improved in such a manner that it is insensitive to pressure fluctuations.
The holder of the present invention exhibits, according to various embodiments of the invention, at least one vibration damper. It absorbs the vibration energy, transferred from the rinse fluid to the workpiece to such an extent that the accuracy of the workpiece position is no longer endangered. Moreover, the working area is not limited by the holder by the invention, because the vibration damper can be attached outside the working area.
If the vibration damper includes two parts, vibrating relative to each other, then these embodiments of the invention employ a first type of vibration damper, which is connected on both ends to the parts and thus dampens the relative movement of the parts to each other. Furthermore, its damping properties can be adjusted. Moreover, the vibration damper is provided with a spring constant. As its damping ability increases, the spring constant also increases. If the holder has such a vibration damper, the natural frequency of the holder increases with the workpiece attached to it. If the frequency of the excited vibration approximates the natural frequency, the vibration damper is adjusted in such a manner that the natural frequency is adjusted to be far away from the frequency of the excited vibration.
If one part is attached, for example, to a fixed wall, and projects from it and its free end bears the workpiece, another embodiment of the invention uses a second type of vibration damper. The excited vibrations subject the part to a pull and pressure load. Therefore, the vibration damper having an inherent damping property, for example in the form of rubber, is mounted on the outside of the part. Another group of appropriate vibration dampers is made of alloys (high damping capacity alloys=HDCM), which are usually based on chromium and iron alloys and can be sprayed expediently under negative pressure on the surface of the part or the holder. The thickness of the sprayed-on or deposited alloy coating can range from 2 to 3 mm.
This second type of vibration damper can be arranged between two parts, i.e., between two steel plates of a clamping device. In this case, the damping material is subjected predominantly to dynamically varying shear loads. Suitable embodiments of a vibration damper of the second type are insulating foils having high damping properties. One example is the insulating foil 2552, marketed by Minnesota Mining and Manufacturing Co., Saint Paul, Minn. Thus, one variation of the invention provides such an insulating foil between the work table and chuck as the special workpiece holder. In another embodiment of the invention, the damping material, for example, rubber, can be vulcanized on the bottom and upper plate of a leveling head, as disclosed in U.S. Pat. No. 5,769,561.
A third type of vibration damper contains a mass, for example, a steel ball, provided with damping material, on which rubber is vulcanized. In an advantageous design of the invention, the rubber body is affixed on the holder in the vicinity of the workpiece. The steel mass can be the holder itself, on whose one side the damping material is affixed. The natural frequency of the rubbery damping material is much lower than that of the holder. Hence, the vibration energy, transferred from the rinse fluid pressure to the holder, is absorbed largely by the damping material. If, on the other hand, the natural frequency of the damping material is about equal to one of the natural frequencies of the holder with the attached workpiece and these natural frequencies are to be damped, it vibrates in phase opposition. That is, it vibrates 180xc2x0 out of phase with the holder, a feature that improves the absorption of the vibration energy of the holder owing to the damping material. One criteria of the invention is to provide for the attachment of an easily adjustable vibration damper.
Another embodiment of the invention provides a fourth type of vibration damper, which absorbs the vibration energy through friction between plates that are connected together and loaded in shear during vibration and of which two or more plates of the same size are screwed or cemented together forming a plate package. Optionally, another plate having a higher coefficient of friction can be inserted between the plates of the package.
Therefore, the invention provides for the holder having different types of vibration dampers depending on the specific design or use of the holder. In this respect, the vibration damper can have advantageously adjustable spring and damping constants.
Another solution of the problem on which the invention is based provides a laminated holder, whose metal sheets are permanently connected together. The adjacent faces of the metal sheets are provided with an expediently higher coefficient on friction. The vibrations exciting the holder cause the metal sheets to move relative to each other. The energy of the relative movement is absorbed by the friction of the metal sheets. The consequence is also a damping of the vibration.
Other embodiments for a holder for exact positioning of a workpiece in the working area of a machine tool, in particular an erosion machine, which exhibits fastening elements for fastening to a stationary support structure and/or fastening or positioning at least temporarily a workpiece at the holder, the invention embodiments provide that there be not only a vibration damper but also at least one area where the holder and the support structure and/or the holder and the workpiece make direct contact. Thus, an attachment of the holder to the support structure and/or to the workpiece is created that is hard to some extent and to some extent designed to damp the vibration by means of the intercalated vibration damper.
In a preferred design of these embodiments of the invention, the area can completely envelop the vibration damper, for example, in the shape of a ring.
In another alternative of these embodiments, there are several spaced areas that are disposed in the four corners in, for example, a holder with a rectangular shape. It is then expedient to screw the holder in the four corner areas together with the support structure and/or the workpiece.
In another design of these invention embodiments, recesses, which receive at least partially the vibration damper, can be formed in at least one of the surfaces that face each other and belong to the holder and/or the support structure and/or the workpiece.
Furthermore, it is advantageous in these latter embodiments for the vibration damper to be made of several sandwich-like connected layers, which exhibit different damping properties. Expediently the effect of the vibration damper can be adjusted, for example, by means of a stationary plate, which is put on the vibration damper and is subjected to the action of adjustable pressure by means of at least one setscrew. Instead of the setscrew, the plate can also be subjected to the action of hydraulic pressure. As an alternative, shims with different damping properties can be inserted.